It is the most disturbing element in the mix that makes Pakistan the most dangerous country in the world: its stockpile of at least 30 and perhaps as many as 45 nuclear weapons. And it is always the element that captures the most attention from US intelligence officials.

The United States has essentially let Pakistan’s nuclear arsenal grow over the past three decades, as succeeding governments in Islamabad have supported US policies in neighboring Afghanistan, first in thwarting the Soviet occupation and then in driving out the Taliban and al-Qaeda.

Still, the fear is that in the chaos that regularly afflicts Pakistan, al-Qaeda or other jihadis will somehow gain control of one of the weapons, some of the highly enriched uranium that forms the core of a bomb or the technology to make a bomb -- or even gain control of the government.

“It’s always been easier to steal a government in Pakistan than to steal a bomb,” said one former senior US intelligence official.

It is not an abstract concern, one driven by war game scenarios. There have been two incidents in the past 20 years that call into question who controls the weapons, controls the technology.

Indeed, the incidents offer chilling precedents to what could happen now in a chaotic Pakistan. One is what Benazir Bhutto called a “nuclear coup” in 1990, while the other is knowledge from intelligence that al-Qaeda’s top leaders, including Osama bin Laden, met with Pakistani nuclear scientists in Afghanistan just before September 11 and offered the terrorist group advice on how to build a crude nuclear device.

For better or worse, the US is confident that it knows where the Pakistani nuclear arsenal is located and that it is secure. And in 2003, the US secretly provided technology and training to Pakistani nuclear scientists so they could develop “permissive action links”—codes that prohibit unauthorized detonation. Prior to US intervention in this area, none of the Pakistani warheads were protected, say US and Pakistani officials.

Moreover, military and intelligence officials have told NBC News that should the need arise, the US is prepared to take out—or simply take—the weapons from Pakistani control. As Condoleezza Rice said at her confirmation hearings in January 2005, “We have noted this problem, and we are prepared to try to deal with it. I would prefer not in open session to talk about this particular issue.”

“There wasn’t much concern about physical security, but a high degree of angst that the government would fall into the hands of bad guys and they would be in charge,” said the former official, who added that there were “some in the nuclear program who are sympathetic to the radicals”.

As laid out in “Critical Mass: The Dangerous Race for Superweapons in a Fragmenting World," a 1994 book by Robert Windrem and William E. Burrows, the first incident unraveled in the summer of 1990 when India and Pakistan were in one of their seemingly innumerable crises. For the first time, the US had detected that Pakistan had actually put together a nuclear weapon without the knowledge of the country’s prime minister, Benazir Bhutto. And not long after Bhutto learned what her military had done, she was deposed by the same men who had kept the weaponization secret from her.

The CIA had determined that in May 1990 Pakistani scientists had succeeded in converting highly enriched uranium from a gas into a heavy metal. The uranium had undergone successive changes, going from gas to pellets to the mold and machined spheres—perfect spheres—that constituted the cores of atomic bombs. The CIA knew that the cores were then stored near the other components needed to make a complete weapon so the Pakistani bomb could be assembled in as little as three hours at Dalbandin, an airbase in the Baluchistan desert well out of reach of Indian jets. There was enough metal to make between six and eight nuclear weapons, each with the explosive capability equivalent to the bombs that devastated Hiroshima and Nagasaki.

The United States later learned the final number of cores was seven. Two cores had been machined in May, and five more were turned out by the end of July. The first two used about 40 pounds of uranium while the last used about 26 pounds each. Like most other things, a learning curve improves efficiency.

The Pakistanis had not only “crossed the line” as the saying went in Washington’s nuclear precincts. They had actually prepared bombs for delivery. More importantly, in relation to the current crisis, the whole scenario had been carried out without Bhutto even knowing what had happened.

“I think it is criminal that the Prime Minister, who is ultimately responsible in the eyes of the people and in the eyes of history, should not be taken into confidence on such a major issue.” She told NBC two years later. “I did not know.”

Bhutto in fact had not just been Prime Minister. She was Defense Minister and Atomic Energy Minister as well.

The decision had been taken by the Army chief of staff, Mirza Aslam Beg, and the country’s president, Ghulam Ishaq Khan. The presidency then, unlike now, was more of a ceremonial post. Both had been proponents of the Pakistani bomb program, which ironically had been started by Bhutto’s father when he had been prime minister. Khan in fact had run the program.

Bhutto also found out in a most unorthodox way. In late June, two long time American friends of hers had come to Islamabad to tell her what happened. Peter Galbraith, then the south Asia specialist on the Senate Foreign Relations Committee, and Mark Siegel, her Washington lobbyist, took her to a garden outside her offices in the Pakistani capital to inform her.

The news Galbraith and Siegel had delivered took Bhutto by surprise, but she knew the consequences. The United States now had the proof it needed to cut off aid to Pakistan under a law called the Pressler Amendment, and ultimately the US did just that.

A few weeks later, the US ambassador delievered the news to her. Robert Oakley informed her that US law required a cutoff in aid to Pakistan if it possessed a “nuclear explosive device” and demanded that Pakistan reverse the process.

Around the same time, US officials flew to Islamabad while Bhutto was on a state visit to the Gulf States to warn Ishaq Khan and Beg there was no way Pakistan could win a war with India and that continued nuclear brinksmanship would risk a catastrophe.

Bhutto, unaware of the US meeting, contacted Ishaq Khan to relay Oakley’s warning and three times called for a meeting of the top-secret committee that ran the nuclear weapons program. Each time Ishaq Khan said he would get back to her. She also asked Beg for an explanation as well and he promised one would be forthcoming.

Neither happened, but on Aug. 6, less than three months after Pakistan had begun the process of building a bomb, Bhutto was deposed. With the world’s attention then focused on Saddam Hussein’s four-day old occupation of Kuwait, Ishaq Khan went on Pakistani television to denounce Bhutto’s government as corrupt and incompetent.

“I have no proof of this,” Bhutto later told NBC News, “but I feel that someone may have turned on the switch in the spring of 1990 to justify the dismissal of my government.” She called it a “nuclear coup.”

More troubling was what former CIA Director George J. Tenet wrote about in his memoir, “At the Center of the Storm” about al-Qaeda’s attempt to obtain nuclear know-how from Pakistani scientists.

In August 2001, just weeks before the 9/11 attacks, two officials of an ostensible Pakistani charity, both senior scientists in the country’s nuclear weapons program, met with Osama bin Ladin and his deputy, Ayman al-Zawahiri, in Afghanistan.

“There, around a campfire, they discussed how al-Qa’ida should go about building a nuclear device,” wrote Tenet.

The scientists were not ordinary scientists. Sultan Bashirrudan Mahmood, was the former director for nuclear power at Pakistan’s Atomic Energy Commission. Chaudiri Andul Majeed, a prominent nuclear engineer, had retired from the Pakistani Institute of Nuclear Science and Technology in 2000. Both institutes were part of the nuclear weapons establishment. Their charity, UTN, also included retired Pakistani nuclear scientists, military officers, engineers, and technicians.

The United States had already learned from Libyan intelligence that UTN scientists had approached Moammar Khaddafi’s government with an offer they thought the Libyans couldn’t refuse: “They tried to sell us a nuclear weapon,” Tenet quoted Musa Kusa, the head of Libyan intelligence, as saying. “Of course, we turned them down.” The CIA was able to confirm through other sources that indeed the offer had been made, according to Tenet.

“CIA passed our information on UTN to our Pakistani colleagues, who quickly hauled in seven board members for questioning,” Tenet wrote, adding with some exasperation, “The investigation was ill-fated from the get-go. The UTN officials all denied wrongdoing and were not properly isolated and questioned.

“In fact, they were allowed to return home after questioning each day. Pakistani intelligence interrogators treated the UTN officials deferentially, with respect befitting their status in Pakistani society. They were seen as men of science, men who had made significant contributions to Pakistan. Our officers read the question etched in the faces of their Pakistani liaison contacts: Surely, such men cannot be terrorists?”

Ultimately, after more intelligence came in, President Bush dispatched Tenet to Islamabad in November 2001 with a file of accusations and a less than subtle threat.

“After a few pleasantries, I explained to President Musharraf that I had been dispatched by the U.S. president to deliver some very serious information to him. I launched into a description of the campfire meeting between Usama bin Ladin, al-Zawahiri, and the UTN leaders. ‘Mr. President,' I said, ‘you cannot imagine the outrage there would be in my country if it were learned that Pakistan is coddling scientists who are helping Bin Ladin acquire a nuclear weapon. Should such a device ever be used, the full fury of the American people would be focused on whoever helped al-Qa’ida in its cause.'”

Musharraf was incredulous.

“But Mr. Tenet, we are talking about men hiding in caves,” Tenet quotes Musharraf as saying. “Perhaps they have dreams of owning such weapons, but my experts assure me that obtaining one is well beyond their reach. We know in Pakistan what is involved in such an achievement.”

“Mr. President, your experts are wrong,” Tenet said he responded.. “I told him that the current state of play between weapon design and construction and the availability of the needed materials made it possible for a few men hidden in a remote location—if they had enough persistence and money, and black enough hearts—to obtain and use a nuclear device.”

A second round of interrogations followed and the full story finally emerged. As Tenet recounts it, there was little doubt that bin Laden and Zawahiri saw Pakistan’s nuclear fraternity as its most likely source of help. Moreover, there was even less doubt of bin Laden’s interest in nuclear weapons.

“Mahmood confirmed all we had heard about the August 2001 meeting with Usama bin Ladin, and even provided a hand-drawn rough bomb design that he had shared with al-Qa’ida leaders. He told his interrogators that he had discussed the practicalities of building a weapon. ‘The most difficult part of the process,’ he told Bin Ladin, ‘is obtaining the necessary fissile material.’ ‘What if we already have the material?’ Bin Ladin replied. This surprised Mahmood. He said he did not know if this was a hypothetical question or if Bin Ladin was seeking a design to use with fissile material or components he had already obtained elsewhere.”

An unidentified senior al-Qaeda leader also present at the campfire displayed a canister for the visitors that may or may not have contained some kind of nuclear material or radioactive source. He also shared his ideas of building a simple firing system for a weapon using commercially available supplies, according to the interrogation quoted by Tenet.

Tenet says in spite of extensive efforts to learn whether bin Laden actually had HEU, the US intelligence and law enforcement community had no luck. Luck in fact may be what is needed more than anything else in dealing with Pakistan’s nuclear weapons.